(1) Field of the Invention
The present invention relates to optical information devices that optically record information signals on optical information recording media (hereinafter, referred to as optical discs) or reproduce information signals recorded on the optical discs, and more particularly, to an optical information processing device suitable for recording or reproduction of information signals on a multilayer optical disc with a plurality of recording layers stacked upon each other, and a tracking control method adopted by the optical information processing device.
(2) Description of the Related Art
Currently commercially available optical discs include, for example, DVDs with a storage capacity of 4.7 GB (Giga Byte) on one layer of one side and Blu-ray Discs having a larger storage capacity.
Recently proposed multilayer optical discs have, in addition to one or two information signal recording layers (hereinafter, the information signal recording layer is simply referred to as “recording layer”, for clarity), a stack of three or more recording layers to increase the storage capacity. Such multilayer optical discs are rapidly proceeding toward standardization and practical utilization.
In order to achieve a large capacity multilayer optical disc, attention is being given to a so-called “grooveless multilayer disc” including a disc layer used exclusively for tracking control (hereinafter, the disc layer used exclusively for tracking control is simply referred to as “guide layer”) in addition to the recording layers. The guide layer is used for detecting a tracking error signal that controls a light spot that is used for recording/reproduction and converged on a recording layer. Only the guide layer has a continuous guide groove formed thereon to detect the tracking error signal.
By the way, an optical pickup suitable for recording and reproduction of such a grooveless multilayer disc is disclosed, for example, in Japanese Patent Application Laid-Open No. 2003-067939 (PTL 1). The pickup in the disclosure adopts a tracking control method in which light beams are converged to form independent light spots on the recording layer and the guide layer, and tracking error signals are detected from the light spots converged on the guide layer (hereinafter, the light spots are referred to as “light converging spots G” for clarity) to perform tracking control of the light converging spots G based on the tracking error signals, while performing tracking control of a signal light spot converged on the recording layer (hereinafter, the light spot is referred to as “light converging spot R” for clarity) so as to follow the light converging spots G.